Clock Settings

STK600 includes several clock options for the target AVR.

A switch selects between the following three options:

Programmable Clock Generator

The programmable clock generator is set from the PC software. The frequency can be set from 1.1kHz to 32MHz with 0.5% accuracy.

To use the programmable clock generator as clock source, set the CLOCK switch to EXT position.

Crystal Oscillator

The on-board crystal oscillator will work with ceramic resonators or crystals between 4 and 24MHz (AT-cut, fundamental, and parallel resonant crystals). Place a crystal in the crystal socket (located next to the PROGRAM button).

To use the crystal oscillator as a clock source, set the CLOCK switch to the XTAL position.

XTAL1 Pin Tri-stated

If the target AVR run on the internal oscillator, the XTAL1 pin can be disconnected from the clock sources on STK600.

To disconnect the XTAL1 pin, set the CLOCK switch to the INT position.

Real Time Clock

The STK600 also features a 32768 Hz oscillator, which can be used to make a real time clock. The output from the oscillator is available on the 32KHz pin on the AUX header. This clock can be routed to the TOSC1 pin on the target AVR by placing a jumper between the 32KHz and TOSC1 pin on the AUX header.

See also Port Connectors for more information about the AUX header.

Other Considerations

High-Voltage Programming

When programming the target AVR in High-Voltage programming mode, the clock settings is overridden and the device is clocked directly from the STK600 controller. The clock selection switch can be set to any position.

On-chip Crystal Oscillator

In a real-life application where the crystal can be placed close to the AVR's XTAL1 and XTAL2 pins, there is no need for an external oscillator circuit. The long clock signal lines and socket system connectors on STK600 makes it difficult to drive a crystal with the on-chip oscillators. This is resolved by having a crystal oscillator on STK600. The oscillator is designed to operate over the full target voltage range.

Shared XTAL1/Port Pin

Some AVR devices have a XTAL1 pin which also can be used as a regular I/O port pin. The routing card for these devices will connect the device pin to both the XTAL1 net and a port pin header on the STK600. Hence, to use the pin as a I/O port the clock selection switch must be set to position INT to disconnect the clock drivers on STK600 from the pin.